In recent years, by employing an electrophotographic full color image forming method using an electrostatic image developing toner, it has become possible to achieve a full color image (a full color print) formation, in addition to a conventional monochrome image (a monochrome print) formation mainly used for producing a document.
In such electrophotographic method for forming an image using a toner, a full color image having a desired color is formed by superimposing plural toner images each made of a yellow toner, a magenta toner, or a cyan toner (for example, refer to Patent document 1).
In particular, when a full color image used for a catalog, an advertisement, or a poster is formed, it is required that an original picture image should be reproduced faithfully. In connection with it, it is required to realize a high degree of gradation or to raise more colorfulness of the color of a picture image, and further it is required to extend the color reproduction range of a picture image.
However, it has become recognized that only three kinds of color toners of yellow, magenta and cyan are insufficient to realize sufficient color reproduction when they are used as electrostatic image developing toners for forming a full color image. Then, there has been proposed a method of performing image formation in which a light color toner is used for the highlight image portion having a high lightness and a deep color toner is used for a solid image of the low lightness image portion, in addition to these three color toners (for example, refer to Patent documents 2 to 7).
For example, in Patent document 2, there was proposed a image forming method in which a light cyan toner having a high lightness and a deep cyan toner having a low lightness are used as electrostatic image toners each specified the hue of the red to green direction and the hue of the yellow to blue direction in addition to a yellow toner and a magenta toner
However, the full color image formed using a light color toner and a deep color toner was unable to produce a sufficient level of color reproduction range which was required. The reason was as follows: since the used light cyan toner was produced by reducing the amount of the pigment which constituted a colorant to exhibit a high lightness, the color saturation of the light color toner was lowered and it was difficult to realize a sufficient color reproduction.